1. Field of the Invention
The present invention relates to a speaker device that includes a magnetic-flux applying unit that applies magnetic flux on a voice coil that is supported in a magnetic gap in a vibratable manner, and a heat-dissipating member for the speaker device.
2. Description of the Related Art
In a so-called outer-magnet speaker device, a center pole extends upward from the center of the speaker device. Annular magnets are disposed around the center pole. A top plate is stacked atop the annular magnet, the top plate and the annular magnets together forming a magnetic circuit. A magnetic gap is formed between the top plate and the center pole. A voice coil, connected to a diaphragm, is supported to vibrate in an axial direction of the center pole in the magnetic gap. When a magnetic flux is applied on the voice coil via the top plate, the voice coil vibrates in response to the sound signal supplied to the voice coil. This vibration of the voice coil causes the diaphragm to vibrate and generate a sound.
Each of the top plate and the center pole has a magnetic-flux applying unit that applies magnetic flux on the voice coil. The top plate, which is disposed outer radially to the voice coil, has an outer-radial magnetic-flux applying unit. The center pole, which is disposed inner radially to the voice coil, has an inner-radial magnetic-flux applying unit.
In such an outer-magnet speaker device, it is essential that the magnetic gap in the vibration direction of the voice coil be at least longer than the vibration stroke. Consequently, it is essential that the surface of the top plate facing the vibration direction (center pole axis direction) be of a certain length. In other words, it is essential that the top plate be of a certain thickness in the axial direction.
Thus, in a conventional speaker, instead of a single top plate, a plurality of top plates of a predetermined thickness in axial direction are stacked one on top of another. The method of achieving the predetermined thickness of the top plate by stacking a plurality of thinner top plates (see Japanese Patent Laid-Open Publication No. 2003-219494) is less expensive than using a single top plate of the required thickness.
Incidentally, in the outer-magnet speaker device having a structure described above, a large amount of heat is generated when the voice coil vibrates in response to the sound signal supplied to it. The heat from the voice coil is conducted to the top plate causing it to become hot. Since the heat is detrimental to the performance and durability of the speaker device, ways have been sought to resolve the problem of heat generation. The problem is more acute, particularly, in a bass speaker device requiring a large supply of current to the voice coil and in an automobile speaker device whose location does not allow quick heat dissipation because of space constraints. The following solutions have been proposed as conventional solutions to the problem.
(1) A method of heat dissipation is proposed in which the heat from the voice coil is conducted to a yoke and therefrom to a frame, and the heat from the frame is dissipated to the exterior (see Published Utility Model Application No. S63-52386).
(2) Another method of heat dissipation is proposed in which a radiator-plate composed of a material with a good thermal conductivity is provided between the top plate and the frame and the heat from the top plate is dissipated to the exterior via the frame (see Published Utility Model Application No. S63-200995).
(3) Yet another method of heat dissipation is proposed where a magnetic circuit cover is provided with an opening for air flow, and the heat from the voice coil escapes to the exterior through this opening (see Published Utility Model Application NO. H5-74094).
However, in the method involving heat dissipation via the yoke and the frame, the heat dissipation efficiency is low as no heat-dissipating unit is provided for the top plate. Hence, the issue is not completely addressed. In the method involving heat dissipation by way of providing the radiator plate between the top plate and the frame, there is no escape of heat through the yoke because the yoke and the frame are not in contact with each other. Hence, again, the issue is not completely addressed. In the method involving heat dissipation from the opening in the magnetic circuit cover, the ability for heat dissipation is not adequate enough. Hence, in this case too, the issue is not completely addressed. Further, in the speaker device having the top plate consisting of a plurality of plates stacked one on top of another provided as a outer-radial magnetic-flux applying unit, the heat from the second plate from the outer surface and those further inward is not easily dissipated, again failing to fully address the issue.